The present invention relates generally to the cancellation of interference signals present in a digital data communication system. More particularly, the present invention relates to the cancellation of periodic interference signals with an adaptive tone predictor.
Digital data communication systems that transfer data over the public switched telephone network (PSTN) are generally known. For example, current modem systems, such as those compatible with the ITU-T Recommendation V0.90, are configured to transfer data at theoretical speeds of up to 56 kbps via a digital PSTN. The actual data rate obtained may vary for the particular communication session depending upon various practical factors such as the quality of the communication channel, the existence of digital impairments (e.g., robbed bit signaling and digital pads), and the amount of interference present in the channel. The interference may include a substantially random colored noise component and a component having substantially periodic characteristics.
The minimum signal to noise ratio for the latest generation of modem systems is higher than the corresponding minimum signal to noise ratio of older systems (e.g., voiceband or V0.34 modems). Consequently, such older systems could tolerate a higher amount of additive noise in the communication channel without a sacrifice in performance. Furthermore, the frequency range of periodic interference signals that may appear in the context of older modem systems makes such interference signals tolerable. For example, periodic interference signals in a typical V0.34 modem system have frequencies above 300 Hz. In contrast, periodic interference signals in a typical V0.90 modem system may have frequencies as low as 60 Hz; such low frequency noise may not be tolerable in a V0.90 system.
In current V0.90 modem systems, the additive noise may have an adverse affect on the function of one or both of the end-to-end devices (e.g., the two modems that communicate over the PSTN). For example, during various initialization procedures, a typical V0.90 modem system will perform initial training of its equalizers, echo cancelers, and other adaptive components. Such training procedures are intended to allow the adaptive components to adjust their performance characteristics in accordance with the current line conditions. After the adaptive components are initially trained, the system eventually begins operating in the normal data tracking mode. In most cases, the adaptive components are periodically updated during the data tracking mode in an attempt to optimize the performance of the data communication system.
The receiver section of a modem device generally includes some type of adaptive equalizer structure, e.g., adaptive digital filters, that are designed to characterize the response of the communication channel established between the two modem devices. However, the adaptive equalizers may not be designed to compensate for periodic interference present in the received signal. Accordingly, during the training process, such periodic interference may remain in the equalized signal. Consequently, the training of the receiver equalizers may be affected by the interference and, therefore, they may not be optimally trained.
Prior art receiver arrangements may include a noise predictor (NP) element for canceling the noise components that are uncompensated by the equalizers. However, conventional finite-length NPs may not have the computational power necessary to cancel periodic interference in addition to other forms of additive noise. Accordingly, NP techniques may also lack the ability to adequately cancel periodic interference from the equalized signal.
Prior art echo cancellation schemes (based on adaptive filtering techniques) may also suffer from the above shortcomings of the prior art. As discussed above, periodic interference may be present in the received data signal. Conventional V0.90 echo cancelers do not compensate for periodic noise during training or data tracking. Thus, the periodic interference component will remain and may therefore affect the echo canceler training. The presence of the periodic interference will adversely affect the signal to noise ratio and may result in a lower effective data rate.
Accordingly, the present invention provides an improved technique for the cancellation of periodic noise in a digital data communication system. The techniques described herein may be utilized in a receiver equalizer context or in an echo canceler context. The present invention also provides a tone prediction scheme that can generate update signals for adaptive components in the communication system. The tone prediction scheme may be implemented in a digital data communication system that is susceptible to periodic interference signals. The techniques of the present invention can be utilized during an initialization mode to enhance the training of adaptive components in the system and/or during the normal tracking mode to maintain an acceptable signal to noise ratio.
The above, and other features of the present invention may be carried out in one form by a method for preconditioning an update signal for an adaptive processing component of a digital data communication system. Such a method may include the steps of receiving a signal containing a periodic interference component, performing a delay operation on the signal to decouple the periodic interference component from the signal, filtering the decoupled periodic signal with an adaptive filter structure to obtain an interference estimate, and generating an update signal in response to the interference estimate. The update signal may be utilized to adjust characteristics of the adaptive processing component.